MRSA, or methicillin-resistant staphylococcus aureus bacteria, was found in 83 percent of the influent — the raw sewage — at all plants tested in Mid-Atlantic and Midwest. However, bacteria-positive samples decreased as treatment progressed. Only one treatment plant showed the bacteria in treated water leaving the plant, and that plant does not regularly use chlorination, a tertiary step in wastewater treatment.

MRSA is well-known for causing difficult-to-treat and potentially fatal bacterial infections in hospital patients, but since the late 1990s it has also been infecting otherwise healthy people in community settings. MRSA lives on healthy people, but most of the time it does not cause symptoms or infection. It can spread to others through human contact.

When infection occurs, it can present itself as a red, swollen, painful area on the skin. It is more likely to show up near hair follicles, like a pimple or pustule, according to the National Institutes of Health. When it spreads to the bloodstream, heart, other organs or area of recent surgery, the symptoms of this severe infection include chest pain, cough or shortness of breath, fatigue, fever and chills, general ill feeling, headache, rash and wounds that do not heal.

“MRSA infections acquired outside of hospital settings ... are on the rise and can be just as severe as hospital-acquired MRSA," said Amy R. Sapkota, assistant professor in the Maryland Institute for Applied Environmental Health and research study leader. "However, we still do not fully understand the potential environmental sources of MRSA or how people in the community come in contact with this microorganism."

Infected people can shed MRSA from their noses, skin and through their feces, so wastewater treatment plants are a likely reservoir for the bacteria. Swedish researchers have previously identified the presence of MRSA in waste-water treatment plants in Sweden, and this new University of Maryland-led study confirms the presence of MRSA in U.S. facilities. The study was published in the November issue of the journal Environmental Health Perspectives.

The research team, including University of Maryland School of Public Health and University of Nebraska Medical Center researchers, collected wastewater samples throughout the treatment process at two Mid-Atlantic and two Midwestern treatment plants. These plants were chosen, in part, because treated effluent discharged from these plants is reused as "reclaimed wastewater" in spray irrigation activities. The researchers were interested in whether MRSA remained in the effluent.

They found that MRSA, as well as a related pathogen, methicillin-susceptible Staphylococcus aureus (MSSA), were present at all four treatment plants, with MRSA in half of all samples and MSSA in 55 percent.

Ninety-three percent of the MRSA strains and 29 percent of MSSA strains were resistant to two or more antibiotics, including several that the U.S. Food and Drug Administration has specifically approved for treating MRSA infections, researchers said. At two plants, MRSA strains showed antibiotic resistance until chlorination was introduced and appeared to eliminate all MRSA. Researchers said this suggests that plants may want to test for increased antibiotic resistance, particularly where chlorination is not employed.

"Our findings raise potential public health concerns for wastewater treatment plant workers and individuals exposed to reclaimed wastewater," said Rachel Rosenberg Goldstein, environmental health doctoral student in the School of Public Health and the study's first author. "Because of increasing use of reclaimed wastewater, further research is needed to evaluate the risk of exposure to antibiotic-resistant bacteria in treated wastewater."